1. Field of the Invention
The present invention relates to heat exchangers that may be used as evaporators in a refrigeration/air conditioning system. It is particularly well suited for use in an automotive vehicle air conditioning system.
2. Description of the Prior Art
Japanese examined utility model (Koukoku) No. 53-32378 discloses a heat exchanger used as an evaporator of the type shown in FIG. 19. It has a plurality of tube-units 510 each formed by a pair of plates 511 and 512 joined to each other. Each tube-unit 510 has a U-shaped tube portion 516 and a first tank portion 515 and a second tank portion 518 disposed at opposite ends of the tube portion. Tube-units 510 are connected to each other with corrugated fins 517 disposed between them. An inlet pipe 501 is joined to the first tank portion 515 disposed at one end of the U-shaped tube for introducing refrigerant therethrough. An outlet pipe 502 is joined to the second tank portion 518 disposed at the other end of the U-shaped tube 516 for allowing refrigerant to flow out from the second tank portion.
FIG. 20 graphically illustrates the relationship between a flow pattern of refrigerant in various evaporator configurations and a temperature gradient (as a function of position along the heat exchanger) of air passed through the heat exchanger when it is used as an evaporator of refrigerant. The refrigerant flow pattern for various structural arrangements of heat exchangers is shown schematically in the upper portions of FIG. 20 and the air temperature just downstream of the heat exchanger is indicated at a lower portion of FIG. 20.
In the evaporator indicated in the "A" portion of FIG. 20, refrigerant introduced into the first tank portion 515 through the inlet pipe 501 flows to the second tank portion 518 through the U-shaped tube portions 516. The temperature of the air gradually decreases from the position close to the inlet pipe to the position close to the outlet pipe.
The evaporator which is indicated in the "B" portion of FIG. 20 has a separate plate 520 in the first tank portion 515. The refrigerant flow into the front portion 515a of the first tank portion 515 through the inlet pipe 501 is interrupted so that the refrigerant flows into the second tank portion 518 through the U-shaped tube 516 which opens to the front portion 515a of the first tank portion 515. The refrigerant introduced into the second tank portion 518 then flows toward the rear portion 515b of the first tank portion 515 through the U-shaped tube 516 which opens to the rear portion. Refrigerant which has flowed into the first tank portion 515 flows out through the outlet pipe 502. The temperature of air gradually decreases from the position close to the inlet pipe 501 to the position close to the separate plate 520. The temperature of air is high at a portion of the evaporator that corresponds to a flow of refrigerant downstream of the separate plate 520 and gradually decreases from the position close to the inlet pipe 501 to the position close to the separate plate 520. The temperature of air is high at the downstream of the separate plate 520 and gradually decreases from the position close to the separate plate 520 to the position close to the outlet pipe 502.
In the evaporator indicated in the "C" portion of FIG. 20, a separate plate 520a is disposed in the first tank portion 515 in order to divide the first tank portion 515 into a front portion 515a and a rear portion 515b and a separate plate 520b is disposed in the second tank portion 518 in order to divide the second tank portion 518 into a front portion 518a and a rear portion 518b. The refrigerant flowed into the tank portion 515 through the inlet pipe 501 is interrupted by the separate plate 520a, so that the refrigerant flows into the front portion 518a of the second tank portion 518 through the U-shaped tube 516. After that the refrigerant flows into the rear portion 515b of the first tank portion 515 through the U-shaped tube 516 which connects the front portion 518a of the second tank portion 518 and the rear portion 515b to the first tank portion 515. The refrigerant flows from the rear portion 515b of the first tank portion 515 to the rear portion 518b of the second tank portion 518 through the U-shaped tube 516 which connects the rear portion 515b of the first tank portion 515 and the rear portion 518b of the second tank portion 518. The temperature of air becomes low at the upstream of the separate plate 520a or the separate plate 520b and becomes high downstream of them.
FIG. 21 is a schematic diagram of the flow pattern of the refrigerant in a conventional evaporator. Refrigerant flows into the tank portion 515 through the inlet pipe 501 in a gas-liquid phase. Mist of the liquid refrigerant is mixed with gas refrigerant. The quantity and velocity of refrigerant flowing in the tank portion and the tube portion increases, especially when the heat exchanging capacity required for the evaporator becomes high. The force of inertia of the liquid refrigerant in tank portion 518 flowing toward the wall shown in the right side of FIG. 21 increases with high velocity flow of refrigerant. The quantity of liquid refrigerant around the inlet port is, therefore, much smaller than that of the liquid refrigerant in front of the wall, namely downstream. A large amount of the liquid refrigerant mixed in the gas refrigerant as a mist flows toward the wall 521 in the tank portion 518 by the force of inertia.
The liquid refrigerant mainly flows into the U-shaped tube portion opening ahead of an end wall of the tank portion and the gas refrigerant mainly flows into the U-shaped tube portion opening around the inlet pipe. Therefore there is an imbalance of distribution of refrigerant flowing into the tube portion. Such imbalance causes the temperature gradient of air output across the width of the evaporator to be uneven.
FIG. 34 is a schematic view of a conventional evaporator. A first tank portion 311 has an inlet port 314 at the left side thereof. One end of each a plurality of tubes 313 is connected to the first tank portion 311 and the other end of each of tubes 313 is connected to a second tank portion 312. The second tank portion 312 has an outlet port 315 at the right side thereof from which refrigerant flows.